KVM got slow after adding more physical memory to host

[Nikola Ciprich <nikola.ciprich@linuxbox.cz>]

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Hello,

I'm trying to solve quite a weird problem on one of our customers' box.
It's quad core Xeon X3430  @ 2.40GHz, running 64bit centos with 2.6.38 (I also tried upgrading to
2.6.39).
After increasing physical memory from 16 to 20GB, all guest got incredibly slow, starting just
one windows or linux guest causes host to be quite slow, and quest seems to be even slower.
hardware virtualisation is on, KVM is used, no relevant update since the update.
here's about 3 seconds of kvm_stat:

 kvm_exit                                    659513  103922
 kvm_entry                                   659505  103920
 kvm_emulate_insn                            631118   99344
 kvm_exit(IO_INSTRUCTION)                    625223   98398
 kvm_userspace_exit                          624979   98359
 kvm_pio                                     624970   98357

but according to atop, there is almost no I/O activity from guests, and running top in them
shows them to really be idle...
but they eat like 60-70% of host CPU core time each..

Do You have any idea on what should I check?
thanks a lot in advance

nik
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Ing. Nikola CIPRICH
LinuxBox.cz, s.r.o.
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